Point people to python3-dev if needed (#3359)

* Point people to python3-dev if needed

* oops, syntax fix

* Fedora/RHEL as well
diff --git a/docs/installation.rst b/docs/installation.rst
index 888b8bc..2c24d35 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -74,14 +74,16 @@
 the OpenSSL and ``libffi`` libraries available on your system.
 
 For Debian and Ubuntu, the following command will ensure that the required
-dependencies are installed:
+dependencies are installed (replace ``python-dev`` with ``python3-dev`` if
+you're using Python 3):
 
 .. code-block:: console
 
     $ sudo apt-get install build-essential libssl-dev libffi-dev python-dev
 
 For Fedora and RHEL-derivatives, the following command will ensure that the
-required dependencies are installed:
+required dependencies are installed (replace ``python-devel`` with
+``python3-devel`` if you're using Python 3):
 
 .. code-block:: console